1. Field of Invention
The inventive concepts disclosed and claimed herein relate generally to a coating composition and, more particularly, but not by way of limitation, to a composition having infrared-reflective clusters of titanium dioxide particles cemented together with a precipitate.
2. Background of the Invention
With high energy costs and environmental concerns, there is continuing effort to reduce air conditioning requirements. An important approach for reducing air conditioning requirements is to use roofing products that reflect solar radiation. Radiation is the transfer of heat through electromagnetic energy, typically solar radiation. Absorption of radiation causes an increase in temperature at the absorbing surface and a transfer of the heat to underlying surfaces. In situations where it is desirable to reduce heating due to solar radiation, surfaces exposed to the solar radiation are treated to reflect or scatter the radiation rather than absorb it.
Solar radiation received on the surface of the earth comprises mostly visible and near-infrared (NIR) wavelengths, with NIR consisting of more than half of the total solar radiation. Visible wavelengths are those from about 400 nm to 700 nm while NIR wavelengths are those from about 700 nm to 2500 nm. Absorption of certain visible wavelengths and reflection of the un-absorbed visible light provides color. Coatings typically include colored pigments to obtain a desired color. For example, the color blue is obtained by using blue pigment to absorb most of the non-blue visible light with wavelengths longer than 460 nm.
Typically, black surfaces and ones having a dark color tend to absorb solar radiation causing an increase in the surface temperature. In contrast, white surfaces tend to reflect or scatter solar radiation, causing much less heating. While white surfaces offer desirable heat absorption properties, in many applications the lack of surface color is aesthetically unpleasing. Since the human eye can not see infrared light, the absorption or reflection of the infrared light by the colorants bears no consequence on the color of the coatings. Thus, a surface coating should ideally absorb only the visible radiation necessary to provide the desired color, and absorb none of the NIR radiation.
Unfortunately, most of the conventional colorants strongly absorb infrared light. The U.S. EPA Energy Star Initiative requires that to get LEED (Leadership in Energy and Environmental Design) credit, the total solar reflectance (TSR) needs to be greater than 78% for a low sloped roof, and greater than 25% for a steep sloped roof. For the most part, products that have a TSR greater than 78% are either white or metallic, which is often not aesthetically pleasing. Darker colored roofing materials have a TSR in the 20 to 30 percent range; therefore, most dark colored residential roofs must be steep sloped. Thus there is a need for economic colorants and coating compositions that can provide high solar reflectance and reduced solar heat absorption, as well as desired color and aesthetic appearance.